This proposal is designed to determine the usefulness of a protective environment in the management of allogeneic marrow transplant recipients. The protective environment consists of sterile laminar air flow (LAF) rooms, sterile diet, oral nonabsorbable antibiotics and skin cleansing. Patients with acute leukemia or aplastic anemia undergoing allogeneic marrow transplants from HLA matched siblings are randomized to the protective environment or to a control group with conventional isolations. The objectives of the study are: (1) to attempt to improve further the effectiveness of the protective environment in preventing bacterial and fungal infection during periods of granulocytopenia, (2) to evaluate the effect of the protective environment in preventing infection after marrow engraftment but during periods of immunosuppression, (3) to evaluate the impact of the protective environment on the incidence and severity of graft versus host disease are more effective in a protective environment, (4) to evaluate the effect of the protective environment in preventing infection in patients who reject the marrow graft, thus making it possible to carry out a reengraftment procedure, (5) to determine the effect of the protective environment on infection and relapse rate following intensive chemotherapy administered to patients with leukemia in relapse prior to marrow transplantation, (6) to define the effect of the protective environment on ultimate survival.